Technically applied polymer membranes for gas separation are usually composite membranes that consist of a porous substructure and pore-free, dense polymer film. For this purpose a composite membrane is utilized or the composite membrane encompasses a corresponding composite material. It is important for the technical usability to achieve high gas flows per area in order to keep the membrane surfaces and the energy costs low. The potential selectivity and gas flow are given by the polymer characteristics. Proportional to increasing layer thickness the flow falls off and the selectivity remains essentially the same. It therefore depends on providing suitable materials and to process these into the smallest layer thicknesses into composite membranes or integrally asymmetric membranes.
The polymer materials available for the manufacture of a separative-selective layer of the composite membranes can be divided into elastomers and glassy polymers that distinguish themselves through the location of the glass transition temperature above or below room temperature. Both polymers are suitable for gas separation. Optimized, technically applied membranes are usually deployed with layer thicknesses of the separative-active layer of 0.5 to 1 μm.
Moreover, the separation of polar gases from non-polar gases or gas mixtures is important in numerous industrial processes, such as, for example, the separation of acidic gases such as carbon dioxide (CO2) or hydrogen sulfide (H2S). For example, the separation of CO2 from natural gas is important in the petrochemical industry since CO2 is present at high concentrations in many natural gas reserves. Especially, carbon dioxide in combination with water is corrosive and can therefore destroy pipelines or other equipment. Furthermore, the presence of carbon dioxide reduces the heating value of natural gas.
For the separation of carbon dioxide from natural gas membrane installations have been used to selectively remove carbon dioxide from gas flows or gas mixtures. In Quadripu, Pakistan, for example, a membrane installation wherein about 14 mio. m3 of natural gas are cleaned per day using asymmetric cellulose acetate membranes. The carbon dioxide flows through the cellulose acetate membrane at a rate of approximately 0.2 m3/(m2 h bar). The selectivity relative to methane is between 15 to 20, the selectivity of the cellulose acetate membrane relative to nitrogen is slightly higher.
For many applications which high pressure is not available the gas flow through a cellulose acetate membrane is too small, so that the required membrane surfaces are very large.
A second important polymer class for membranes for carbon dioxide separation is polyimides. The commercial available Matrimid is an example of this. Known polyimide membranes are produced or distributed for example by UBE in Japan and Air Products in U.S.A. The selectivity of polyimide membranes is higher than the selectivity of cellulose acetate membranes (about 35 to 50 for carbon dioxide/nitrogen), where the flow for polyimide-membranes is similar.
In table 1 membrane materials are listed that are well suitable for carbon dioxide separation from gas mixtures.
Permeabilityselectivityof CO2/Selectivity αPolymer[Barrer]CO2/N2CO2/CH4CO2/H2PDMS30709.63.84.3Pebax ® 2533221233.7Pebax ® 40117860157.8(now Pebax ® 1657)Matrimid10.733420.38Cellulose acetate6.330302.4Permeability in Barrer (1 Barrer = 10−10 cm3(STP) cm/(cm2 s cmHg);(PDMS = Polydi-methyl siloxane)
Table 1 Membrane Materials for Carbon Dioxide Separation
In the case of the polymer listed in table 1 under the label Pebax® (by ARKEMA) it concerns a commercially available multi-block copolymer whose blocks contain polyethylene oxide and Nylon (polyamide). Pebax® MH1657 consists of 60 wt % PEO (polyethylene oxide) and 40 wt % Nylon (PA 6).
Furthermore, it can be gathered from table 1 that for carbon dioxide a membrane made of Pebax® has a significantly higher permeability than a membrane of cellulose acetate or polyimide or Matrimid.
Moreover, U.S. Pat. No. 4,963,265 discloses the manufacture of a composite membrane from Pebax®, wherein the therein described composite membrane has a three times higher flow than the above named membranes.
Further, it has been shown in a scientific presentation, PEG modified poly(amide-b-ethylene oxide) membranes for CO2 separation” (Journal of Membrane Science 307 (2008), 88-95) that the permeability of carbon dioxide can be significantly increased if polyethylene glycol (PEG) is admixed to a membrane polymer made of Pebax®.
In table 2 the properties of Pebax® as well as Pebax®/PEG mixtures are presented.
TABLE 2CO2 permeability and selectivity of Pebax ® MH1657 and mixtures withPEG (polyethylene glycol), measured at 30° C. (comparative data fromJournal of Membrane Science 307 (2008), 88-95)αaPbDcSααCO2/SampleCO2CO2CO2CO2/H2CO2/N2CH4Pebax ®24.8 (73)4.65.39.14515.6Pebax ®/PEG1025.6 (75)4.95.29.24715.8Pebax ®/PEG2027.2 (80)5.15.49.24515.9Pebax ®/PEG3035.8(105)6.25.89.64315.1Pebax ®/PEG4044.9(132)8.05.610.04415.1Pebax ®/PEG5051.3(151)9.65.310.84715.5aPermeability coefficient P in [10−15 mol m/(m2 s Pa)], (Barrer)bDiffusion coefficient D in [10−11 (m2/s)]cSolubility coefficient S in [10−4 mol (STP)/(m3 Pa)]
The values indicated behind PEG in table 2, provide respectively, the weight % of PEG (polyethylene glycol) in the corresponding mixture.
It is one object of the invention to provide a better membrane for the separation of, in particular polar, gases from gas mixtures that exhibit in contrast to the polymer membranes known so far a higher permeability, preferably in regard to carbon dioxide